The present invention relates to textured catalysts and methods of making textured catalysts. The invention also relates to the use of textured catalysts as catalysts for reactions conducted in hydrothermal conditions.
For many years there has been active and increasing interest in conducting chemical processing in aqueous media. In many cases, a potential feedstock is produced along with water. This occurs, for example, in the commercial maleic anhydride process. Alternatively, in the case of fermentations, potential feedstocks are themselves produced in water. Removal of water from these compositions would be time-consuming and costly. Additionally, water has many advantages over more conventional solvents that present problems with toxicity and difficulties with handling and disposal.
On the other hand, water is a relatively reactive medium and most conventional catalysts would quickly become deactivated. To overcome this problem, several workers have conducted aqueous phase reactions over carbon-supported catalysts. For example, Olsen in U.S. Pat. No. 4,812,464 described certain aqueous phase hydrogenations over a palladium on carbon catalyst. Schwartz in U.S. Pat. No. 5,478,952 described the use of catalysts composed of highly dispersed ruthenium and rhenium on a carbon support for catalyzing aqueous phase hydrogenations.
Unlike many oxide supports, carbon supports can maintain their integrity under aqueous, acidic or basic reaction conditions. Carbon supports are also available with exceptionally high surface areas allowing preparation of catalysts with a high dispersion of active metals. Unfortunately, much of the surface area is contained within small micropores, which can result in severe mass transport limitations. As a consequence, many carbon supports are prepared as very fine powders to minimize mass transport effects due to pore diffusion limitations.
It has been found that many catalytic metals, once reduced, are not tightly bound to a carbon surface. During operation, these loosely bound catalytic metals can sinter, or agglomerate, thereby greatly reducing the available catalytic surface area. To lessen the effects of sintering, manufacturers typically limit the amount of catalytically-active metal to less than 1 weight percent of the total catalyst. While this results in a more efficient use of catalytic metal, it requires a larger bed to achieve the required conversions.
To improve properties of carbon-based catalysts, Heineke et al., in Canadian Patent No. 2,191,360, described certain carbon-based catalysts having a titania coating. In the invention of Heineke et al., a carbon support is treated with a Ti or Zr alkoxide, halide or mixed alkoxide-halide. Suitable carbon supports are generally suspended graphite or activated charcoal. Treatment of the carbon support with the Ti or Zr compound is preferably carried out in anhydrous solvents. The reaction is terminated by quenching with a small amount of water. The catalysts are then prepared by precipitating platinum in an aqueous medium using a reducing agent. In the examples, Heineke et al. suspended graphite particles in dry n-butanol and added titanium tetraisopropoxide or titanium tetrachloride followed by stirring for 2 days. Then water was added and the resulting particles are filtered off and dried. The resulting material was treated with an aqueous solution of hexachloroplatinic acid at a pH of 2.75. The platinum was precipitated by addition of sodium formate. The catalysts were tested in the hydrogenation of NO to NH2OH. Compared with catalysts lacking the titania layer, Heineke""s catalysts showed better NH2OH selectivity (86.77 and 91.96 vs. 86.36 and 89.90) and space-time yield (0.798 and 0.897 vs. 0.788 and 0.870).
Despite these, and many other efforts, there remains a need for catalysts having new properties, especially catalysts that are stable in aqueous phase conditions. There also remains a need for new and cost-effective methods of making catalysts. There further remains a need for new aqueous phase catalytic reactions.
In a first aspect, the invention provides a textured catalyst comprising a hydrothermally-stable, porous support comprising a porous interior and an exterior surface; a metal oxide; and a catalyst component. The porous support has a minimum, smallest dimension of at least about 100 xcexcm. Viewed in cross-section, at least about 70% of the catalyst component is within about 5 xcexcm of the minimum cross-sectional area that encompasses about 80% of the metal oxide. Also, at least about 5% of the catalyst component is at least about 10 xcexcm from the exterior of the support.
In a second aspect, the invention provides a method of making a catalyst. This method includes: providing a porous, hydrothermally-stable support; forming a sol comprising metal oxide particles; adding a catalyst component; and drying. The porous, hydrothermally-stable support is directly contacted with a sol comprising metal oxide particles.
The invention also provides a method of conducting a catalyzed reaction under hydrothermal conditions. In this method, at least one reactant passes into a reaction chamber. The reactant is in an aqueous solution. A catalyst is present in the reaction chamber. This catalyst was made by forming a sol of a metal oxide at a pH that is within 2 of the pH of the aqueous solution; adding a catalyst metal; and depositing the metal oxide and the catalyst metal onto a porous, hydrothermally-stable support. The at least one reactant is reacted in the presence of the catalyst and under hydrothermal conditions. At least one product is obtained from the reaction chamber. The product obtained has a higher purity or a higher yield than the product obtained in a comparative test under similar conditions except where the reaction is conducted at a pH that is 4 or greater than the pH of the aqueous solution.
The invention further provides a method of conducting a catalyzed reaction under hydrothermal conditions, comprising: passing at least one reactant into a reaction chamber; reacting the at least one reactant in the presence of the catalyst and under hydrothermal conditions; and obtaining at least one product from the reaction chamber. In this method, the reactant is in an aqueous solution. A catalyst is present in the reaction chamber. The catalyst includes: a porous, hydrothermally-stable support; a metal oxide disposed on the support; and a catalyst component.
Catalysts of the invention (which may be termed xe2x80x9ctexturedxe2x80x9d catalysts because the metal oxide imparts additional xe2x80x9ctexturexe2x80x9d to the porous substrate) can be active and stable, even in aqueous conditions. The textured catalysts can also offer other advantages, such as selectivity. Compared to conventional catalysts and reactions using conventional catalysts, the inventive catalysts and reactions can exhibit unexpectedly superior properties. For example, the inventive catalysts having a metal oxide coating on activated carbon can provide (a) better dispersion of the active metal on the catalyst surface, (b) better stability of the active metal on the catalyst (as opposed to putting the active metal directly on the carbon), and (c) enhanced activity and selectivity. The texturing agent may also be responsible for distributing active components primarily in larger pores such that reactions of substrate are substantially excluded from micropores, reducing diffusion limitations. If a substrate were to react in deeper pores, products that are easily susceptible to over-reactions may be less able to quickly diffuse out of the catalyst and be subject to continued interaction with active metal sites producing unwanted byproducts. The texturing agent may also have the effect of ensuring that the higher value catalytic metals are preferentially partitioned to the easily accessible surface area over the deeper, less accessible pores, thereby requiring less metal to achieve equivalent catalytic activity over standard catalysts.